The present invention relates to three-phase power conditioning filters and, in particular, to zero phase sequence filters.
Harmonic-producing loads are becoming an increasingly large portion of the electrical load in many buildings and installations. Non-linear loads such as the switching mode power supplies found in personal computers can result in substantial harmonic content in a building's electrical distribution system.
Particularly troublesome are the zero phase sequence harmonics introduced by single phase switching mode power supplies. These zero phase harmonics (3rd, 9th, 15th, . . . ) are so called because they are all in phase with each other, no matter which of the three electrical power phase conductors they are on. As a result, their currents are additive on the neutral conductor.
Together with unbalanced portions of the fundamental and other harmonic currents, the resulting current in the neutral conductor can overload the neutral conductor, easily surpassing the current in each of the three phase conductors.
In addition to overloading the neutral conductors, this results in high common-mode noise level (neutral to ground voltage), increased total harmonic distortion level, voltage imbalance, increased losses, etc.
Non-linear loads not only generate zero phase sequence harmonics, they also create 5th, 7th, 11th, 13th, . . . harmonics. These harmonics also contribute to distortion and overall degradation of the distribution system.
Various techniques have been used to try to mitigate these problems.
For zero phase harmonics, zero phase sequence filters have been used. Unfortunately, these filters attract zero sequence currents from not only the load (downstream) side of the filter, but also from the source (upstream) side. This can easily result in a overloaded filter and/or an overloaded feeder neutral conductor despite the fact that the filter is correctly sized for the expected loads. Often the designer has no control over the upstream zero sequence currents and must substantially over-specify the filter.
Prior art zero sequence filters also significantly increase single phase fault levels. They also do not provide any relief from any non-zero phase sequence harmonics.
To reduce their level phase-shifting transformers (typically 1:1) can be employed, but are expensive.
Unless low impedance (and very expensive) transformers are used, these transformers introduce relatively high impedance into the power system. As a result, voltage total harmonic distortion increases.